In recent years, as a method of long-distance transport of crude oil and natural gas, the importance of line pipe has been rising. In particular, submarine line pipes traversing the seas have reached depths of as much as 3000 meters. In general, in the design of pipelines, first the inside diameter of the steel pipe is determined by the amount of fluid transport, then the thickness and grade are determined while considering the crack propagation characteristics for keeping the stress in the circumferential direction at the time of application of internal pressure at a predetermined value and the corrosion loss. However, along with the greater depths reached, the water pressure rises. The collapse strength, which had not been considered that important in the past, is now becoming an issue. The collapse strength is correlated with the ratio of the outside diameter and thickness. By raising the collapse strength of steel pipe, it becomes possible to make the diameter greater and the thickness smaller. Therefore, the collapse strength has begun to become a main design factor determining the size of steel pipe.
However, the collapse strength of steel pipe has been studied for a long time now in oilfield pipes. Statistically a large number of experimental formula have been proposed. Among these, the outside diameter/thickness ratio, yield strength, circularity, thickness evenness, and residual stress have been considered the main governing factors. These studies have mainly been performed for seamless steel pipe of uniform grade, so it was not necessary to discuss the anisotropy of materials much.
However, in trunkline pipe used for long-distance transport, since the diameter is large, steel-pipe made by the method of production of the UOE process has been used. The process of production of steel pipe of the UOE process, as shown in FIG. 1, is comprised of a process of C-ing (pressing), U-ing (pressing), O-ing (pressing), seam welding, and expansion. Further, in the O-ing, as shown in FIG. 2, the diameter is reduced by an O-mold. This is called “upset” of O-ing. Further, expansion is a process of correcting the circularity by pushing outward from the inside by metal segments. Tensile stress in the circumferential direction is applied for plastic deformation. These bending, compression, and tension forming are all performed cold, so the final product, that is, the steel pipe, is given anisotropy in the mechanical properties by the combination of the work hardening and the Bauschinger effect. Note that the Bauschinger effect is the phenomenon of the yield strength dropping in the opposite direction to plastic strain given to a material. Therefore, UOE steel pipe given plastic strain of the tensile direction in the circumferential direction falls in compression yield strength in the circumferential direction, that is, the yield strength with respect to application of outside pressure, due to the Bauschinger effect.
On the other hand, since the direction of application of the main strain at the time of forming perpendicularly intersects the application of load in the axial direction, there is little difference in the stress behavior between application of tension and compression in the axial direction. Further, when the application of load in the circumferential direction is tensile stress, that is, if designing the strength based on the value obtained from a total thickness tensile test for the application of inside pressure, this issue does not arise.
However, in recent years, demand for UOE steel pipe able to be applied to deep sea line pipe has been rising. The collapse strength of steel pipe due to outside pressure has begun to become an issue. Collapse is a phenomenon where the steel pipe is crushed by outside pressure and is a type of buckling, so the yield strength of compression determines the collapse strength. Therefore, when applying UOE steel pipe to line pipe where collapse strength is required, the drop in the compression strength in the circumferential direction due to the Bauschinger effect becomes an issue.
To deal with such issues, the method of using a gas burner to heat steel pipe to at least 150° C. after expansion has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-3545. However, even if performing such heating, it does not mean that the collapse strength of all UOE steel pipe will be improved by 20% or more. Steel pipe with small recovery from the Bauschinger effect due to heating is also seen. Further, for the method of restoring the drop in the compression yield strength by heat treatment, Japanese Unexamined Patent Publication (Kokai) No. 9-49025 discloses and reports numerous research papers. In these methods, the compression yield strength dropping due to forming is restored to the level of the plate material before forming and the collapse strength of UOE steel pipe is improved to a certain extent.
However, if using welding heat or a heat treatment furnace to heat the steel pipe as a whole, in the process of production by the UOE process, work hardening due to compressive strain occurring from the inside surface of the steel pipe to the center of thickness (hereinafter referred to as the “inside surface side) is also lost.”Therefore, the method of improvement of the collapse strength by uniformly heat treating UOE steel pipe as a whole cannot be said to be extremely effective.